1. Field of the Invention
The present invention relates to a mounted circuit substrate and a method for fabricating the same; and more particularly, the invention relates to a mounted circuit substrate characterized by the structure of and the fabrication method for surface layer pads that can withstand pad erosion by molten solder applied over a plurality of times for component replacement on an MCM substrate to which functional components such as LSIs, capacitors, and resistors are mounted by soldering, and also relates to a method for fabricating such a mounted circuit substrate.
2. Description of the Related Art
Recently, thin-film interconnect substrates generally known as MCM (Multi-Chip Module) substrates, which can provide high density interconnections, have been commercially implemented and used in electronic apparatuses such as computers.
With such MCM substrates, since polyimide capable of forming a very thin dielectric film by spin coating is usually used as dielectric material, and since signal patterns are formed by means of sputtering and etching techniques using high-sensitivity resist, high-density patterns never attainable with ordinary printed wiring boards can be achieved.
MCM called MCM-D (Deposit) or MCM-C/D (Cofire/Deposit) is fabricated by forming thin-film circuits in multiple layers by deposition, more specifically, by vacuum deposition process, on a single ceramic substrate or a ceramic circuit within which a thick-film circuit is formed, and many products have been developed and commercialized by various companies.
Between them, MCM-C/D offers significant performance advantages since multi-terminal mounting can be achieved by mounting functional components such as LSIs on one side and input/output terminals on the reverse side, and since, electrically, interconnect paths can be made shorter.
In this case, functional components such as LSIs are mounted by soldering to component mounting pads formed on the surface layer of the MCM substrate; in particular, in the case of LSIs, C4 interconnection (Controlled Collapsed Chip Connection) is employed in which solder bumps are formed on the terminals and LSIs are mounted face down on the MCM substrate.
However, as the number of chips mounted on an MCM substrate increases, the need to replace components, either to repair the components due to C4 interconnection failures or to upgrade LSI functions, is increasing, and more and more stringent demands are being placed on component rework techniques so that component replacement can be accomplished reliably a plurality of times on the same MCM substrate.
That is, component rework techniques are becoming important for component replacement that becomes necessary for such reasons as component failures at test or in use, design changes due to the discovery of bugs in LSIs, or functional upgrades of the LSIs built in the apparatus used.
For example, when mounting a bare-chip component, the chip by itself can be tested for functions after burn in, but when it is mounted on an MCM, its functional compatibility with other functional component chips must also be considered; therefore, if there is a fault in inter-chip transfer timing, the component must be replaced.
In the component rework process, when remounting a solder-joined component such as a C4-joined one, first the defective or old component mounted on the MCM is removed by melting the solder, and then the new or upgraded component is remounted by melting the solder bumps.
The prior art and its associated problem will be described in detail later with reference to accompanying drawings.